1. Field of the Invention
This invention relates to a method for detecting light emitted by two surfaces of a stimulable phosphor sheet, on which a radiation image has been stored.
2. Description of the Prior Art
When certain kinds of phosphors are exposed to radiation such as X-rays, .alpha.-rays, .beta.-rays, .gamma.-rays, cathode rays or ultraviolet rays, they store part of the energy of the radiation. Then, when the phosphor which has been exposed to the radiation is exposed to stimulating rays, such as visible light, light is emitted by the phosphor in proportion to the amount of energy stored thereon during its exposure to the radiation. A phosphor exhibiting such properties is referred to as a stimulable phosphor.
As disclosed in U.S. Pat. Nos. 4,258,264, 4,276,473, 4,315,318, 4,387,428, and Japanese Unexamined Patent Publication No. 56(1981)-11395, it has been proposed to use stimulable phosphors in radiation image recording and reproducing systems. Specifically, a radiation image of an object, such as the human body, is stored on a sheet provided with a layer of the stimulable phosphor (hereinafter referred to as a stimulable phosphor sheet). The stimulable phosphor sheet is then scanned with stimulating rays, such as a laser beam, which cause it to emit light in proportion to the amount of energy stored thereon during its exposure to the radiation. The light emitted by the stimulable phosphor sheet when it is exposed to the stimulating rays is photoelectrically detected and converted into an electric image signal. The image signal is then processed and used during the reproduction of the radiation image of the object as a visible image, which has good image quality and can serve as an effective tool in, particularly, the efficient and accurate diagnosis of an illness. The ultimately obtained visible image may be reproduced as a hard copy or may be reproduced on a display device, such as a cathode ray tube (CRT) display device.
As a method for detecting light emitted by a stimulable phosphor sheet, a method has heretofore been known wherein a photoelectric read-out means for photoelectrically detecting the light emitted by the stimulable phosphor sheet is located only on the side of the stimulable phosphor sheet that is being scanned with the stimulating rays. The light emitted by one surface of the stimulable phosphor sheet is photoelectrically detected by the photoelectric read-out means. Also, a method for detecting light emitted by two surfaces of a stimulable phosphor sheet has been proposed in, for example, U.S. Pat. No. 4,346,295. With the proposed method for detecting light emitted by two surfaces of a stimulable phosphor sheet, two photoelectric read-out means are located on opposite sides of the stimulable phosphor sheet in order to photoelectrically detect the light emitted by the two surfaces of the stimulable phosphor sheet.
The method for detecting light emitted by two surfaces of a stimulable phosphor sheet has the advantages in that the efficiency, with which the light emitted by the stimulable phosphor sheet is guided and detected, can be kept high as a whole, and a high signal-to-noise (S/N) ratio can be obtained.
U.S. Pat. No. 4,394,581 discloses a technique wherein a radiation image storage panel (or a stimulable phosphor sheet) is colored with a coloring agent capable of selectively absorbing the stimulating rays such that an image having high sharpness can be obtained. In this publication, various embodiments are described wherein the layers constituting the radiation image storage panel, such as a phosphor layer, a protective layer, and a substrate, are colored. However, this publication does not describe anything about how the radiation image storage panel should be colored when the operation for detecting light emitted by two surfaces of the radiation image storage panel is to be carried out, nor does it suggest anything about how the radiation image storage panel should be colored when the operation for detecting light emitted by two surfaces of the radiation image storage panel is to be carried out.
Japanese Unexamined Patent Publication No. 2(1990)-112800 discloses a radiation image storage panel wherein a protective layer of the radiation image storage panel (or a stimulable phosphor sheet) is constituted of a material having fine light guiding path structures, which extend in a direction substantially normal to the surface of a phosphor layer, such that an image having good image quality can be obtained. However, this publication does not describe anything about the operation for photoelectrically detecting light emitted by two surfaces of the radiation image storage panel, nor does it suggest anything about the operation for detecting light emitted by two surfaces of the radiation image storage panel.
During the operation for detecting light emitted by two surfaces of a stimulable phosphor sheet, the problems occur in that the stimulating rays, which are scanning the stimulable phosphor sheet, pass through the stimulable phosphor sheet, impinge upon the photoelectric read-out means located facing the back surface of the stimulable phosphor sheet, and are reflected by the photoelectric read-out means. Specifically, the stimulating rays, which have been reflected and diffused by the photoelectric read-out means, impinge upon the portions of the stimulable phosphor sheet other than the portion that is being scanned, and cause these portions of the stimulable phosphor sheet to emit light. Therefore, image information, which is not to be detected at a given instant, is photoelectrically detected at the given instant from the two surfaces of the stimulable phosphor sheet. As a result, the sharpness of the ultimately obtained image cannot be kept high.